Abstract: Rice roots can excrete O2 to surrounding soil and this can convert ammonium(NH4+) to nitrate(NO3-) in the process of nitrification at root surface or in rhizosphere and rice roots are actually always exposed to a mixture of NH4+ and NO3-,although the predominant species of mineral nitrogen(N) in the soil collected from the lowland rice field is NH4+.Therefore,NO-3- nutrition on the surface of rice roots is an important part of N nutrition of rice plants.Two experiments,including whole plant root culture and root-split culture in NH4+ and NH4+/NO3-(75/25) solutions,were conducted to study the effect of enhanced NO3- nutrition on root growth of rice cultivars with different N use efficiency(NUE) during whole growth stage.Two rice cultivars,namely Nanguang(a high NUE rice cultivar) and Elio(a low NUE rice cultivar) selected from 2003 and 2004 field experiments were used in the two experiments.Whole plant root culture indicated that differences existed in the root biomass accumulation between the two rice cultivars when rice roots were exposed to a mixture of NH4+ and NO3-.Compared to NH4+ solution,enhanced NO3- nutrition increased root biomass accumulation of Nanguang during different growth stages while no difference occurred in Elio.Root-split culture showed that the growth of rice roots was stimulated by localized NO3- supply.For example,localized NO3- nutrition increased root biomass accumulation and N accumulation of localized roots in Nanguang cultivar by about 33% and 41%.Moreover,the number of laberal root,surface area and volume of localized roots were also significantly increased in Nanguang cultivar.It is concluded that high N use efficiency rice cultivar was more responsive to NO3- nutrition than low NUE cultivar,implying that enhanced growth of rice roots by NO3- may be one of physiological mechanisms of high nitrogen use efficiency in rice plants.